After I had a bad experience the first time I was installing Service Pack 2 for Exchange 2010 (see here), I have created a small check list before I install SP2 on an Exchange 2010 server.
Unregister Forefront with this command:
C:Program Files (x86)Microsoft Forefront Protection for Exchange Server>FSCUtility.exe /disable
Set the Powershell Execution policy to undefined with these commands:
set-executionpolicy -scope localmachine -executionpolicy undefined
(See http://support.microsoft.com/kb/947821 for details about this.)
From Powershell install the new requriment for the IIS6 WMI Compatibility role.
Restart the server.
Install SP2, and reboot the server again when it is finished.
Register the forefront again with this command:
C:Program Files (x86)Microsoft Forefront Protection for Exchange Server>FSCUtility.exe /enable
Set the Powershell execution policy back:
set-executionpolicy -scope localmachine -executionpolicy Unrestricted
If you are installing the service pack on a DAG cluster, then make sure to set the server in maintenance mode with the StartDagServerMaintenance.ps1 script. See this great blog entry from Tony Redmond http://thoughtsofanidlemind.wordpress.com/2011/12/05/installing-exchange-2010-sp2/